portage/sync/controller.py: Make a repo.postsync.d directory

This then runs per-repo postsync hooks only on scripts in the repo.postsync.d directory.
This also maintains compatibility with existing scripts in the postsync.d dir or other
sub-directories.
Both postsync.d directories support subdirectories.
Scripts are run in sorted order.
Install a verbose example repo.postsync.d script

Brian Dolbec <dolsen@gentoo.org>: Move the example to repo.postsync.d.
Remove the final hook run else section for the new independant repo.postsync.d directory.
Reword comments for the changes.
bin/ebuild: fix --color=n for bug #531690

When the NOCOLOR variable is overridden in portage.settings, it's
necessary to call the config.backup_changes method in order for the
change to persist through config.reset calls.

X-Gentoo-Bug: 531690
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=531690
Acked-by: Brian Dolbec <dolsen@gentoo.org>